Apparatus and method for modifying wound wire devices

ABSTRACT

A method and apparatus for removing unwanted turns from wound wire segments first brazes foil strips to the unwanted turns at opposed points on the periphery and next passes a high energy electrical discharge between the foil strips. The electrical discharge is of a magnitude and duration sufficient to vaporize the turns of wire between the foil strips. Any unvaporized particles of wire are attached to the foil and can be easily removed. If two pairs of opposing electrodes are positioned adjacent each other, one of the electrodes of each pair can be used for the brazing step and both are used for the vaporizing step.

United States Patent [191 Baker et al.

APPARATUS AND METHOD FOR MODIFYING WOUND WIRE DEVICES Sept. 25, 19733,573,422 4/1971 Langenbach 219/91 2,994,244 8/1961 Hay 219/68 X PrimaryExaminerR. F. Staubly Assistant ExaminerL. A. Schutzman Att0rneyMarvinI-I. Kleinberg [5 7] ABSTRACT A method and apparatus for removingunwanted turns from wound wire segments first brazes foil strips to theunwanted turns at opposed points on the periphery and next passes a highenergy electrical discharge between the foil strips. The electricaldischarge is of a magnitude and duration sufficient to vaporize theturns of wire between the foil strips. Any unvaporized particles of wireare attached to the foil and can be easily removed. If two pairs ofopposing electrodes are positioned adjacent each other, one of theelectrodes of each pair can be used for the brazing step and both areused for the vaporizing step.

7 Claims, 8 Drawing Figures [75 l Inventors: Herbert Baker, Marina DelRey;

David S. Rathje, Culver City; William T. Butek, Jr., Costa Mesa, .7 alQfQalL -W {73] Assignee: Solatron Enterprises, Inglewood,

Calif.

[22] Filed: May 15, 1972 [21] Appl. No.: 253,282

[52] U.S.Cl 2l9/68,2l9/9l,2l9/l17 [51] Int. Cl 823p 1/00 [58] Field ofSearch 219/68, 86', 89, 91, 219/1 17 [56] References Cited I UNITEDSTATES PATENTS 3,431,384 3/1969 Cooper 219/68 3,553,417 l/l971 Smith .l

POWER P0605? :appz V Pawn.

PATENTED 3.761.572

SHEET 10F 2 ,/',?0 Paws? SAZPPA) I F 7 xix?! 12 21 MN, P0405? N & 14Sara? W l 48 J0 50 d f7! 7 APPARATUS AND METHOD FOR MODIFYING WOUND WIREDEVICES The present invention relates to wound wire devices, and, moreparticularly, to a process and apparatus for removing unwanted turns offine wire from a wound wire device.

Many uses are known for wound wire devices. For example, potentiometersare produced by winding a very fine, resistance wire about anappropriate mandrel and, through careful control of the windingoperation, a desired resistance per unit length can be achieved.Apparatus for continuously winding potentiometer coils at high speed hasbeen disclosed and claimed in US. Pat. No. 3,498,567 to H. Baker and D.Rathje. Other uses known for wound wire devices include musicalinstrument strings, cables and the like.

In the precision potentiometer field, however, many problems areencountered in preparing an individual wound wire element forincorporation into a potentiometer device. The wire wound mandrel is, astaught in the above patent, produced in a substantially continuouswinding having predetermined resistance characteristics. This windingcan then be cut into fixed lengths which are connected as apotentiometer device. In such an application, it is desirable to removeturns from the end of the mandrel or from between adjacent turns so asto provide fixed increments of resistance or appropriate terminations towhich terminals may be attached.

In the prior art, the preparation of such wound wire segments for use asa potentiometer resistance element has been accomplished by having aworker with tweezers and a magnifying glass, laborously unwind theundesired turns of fine wire and cut or break the wire with the tweezerswhen the process is completed. It is then necessary to provide somemeans to prevent further unwrapping of the fine wire and to assure agood contact to a terminal.

The manual method described above has proved to be less thansatisfactory although it is widely employed. Since human labor isrelatively expensive, the costs of handling each element can besubstantial and the process is a time-consuming one which, if notcarefully done, can result in a high rejection rate of the finisheddevices.

According to the present invention, it has been found that undesiredturns of the wound wire device can be removed by applying a high energyelectrical discharge through the fine wires, which vaporizes orotherwise destroys the unwanted wire. This is accomplished by placingthe wound wire device between opposing electrodes of a resistance typewelder. When a high energy electrical discharge takes place between theelectrodes, the unwanted turns conduct electrical current until theinternal resistance becomes so great that the wires disintergrate orvaporize. Because the wire is relatively fine, virtually the entire spanof wire between the electrodes is consumed. Whatever pieces of wireremain under theelectrodes can be easily removed.

It has also been discovered that the procedure can be improved upon byproviding an intermediate foil strip between the electrodes and thewinding. The foil strip may be a laminate of a less than optimumconductor of electricity such as stainless steel or Kovar or the like,to which has been laminated, plated or otherwise applied, a layer ofgold solder or other low melting point i material. The low melting pointmetal is used to weld or braze the undesired turns to the strip.

When a high energy electrical discharge is applied between theelectrodes, through the strips, the windings are vaporized as before butthe remaining fragments of wire become embedded in the foil strip andtherefore can be removed from the device. in addition,

the use of the intermediate strip tends to protect the weldingelectrodes, minimizing erosion and reducing the necessity for frequencydressing and cleaning of the electrode tips.

A preferred embodiment of the invention and method has been developedwhich employs two different welding techniques. A first and second pairof opposing electrodes are provided and are positioned closely adjacenteach other. The foil strip underlies one electrode of each pair and aparallel gap welding technique is used to bond or braze the strip to theunwanted turns immediately underlying the strip. Because of the limitedelectrical energy employed in parallel gap welding, the strip is heatedonly enough to braze or solder the strip to the wires. This serves thetwo-fold function of attaching the strip to the wires and improving theelectrical path from the electrode into the unwanted wire.

A similar arrangement is provided on the opposite side of the mandrel.The opposite electrodes of each pair are operated in a parallel gapwelding step to bond a strip to the unwanted turns on the opposite sideof the mandrel, as well. The electrodes of each pair are then connectedto opposite terminals of a resistance welding power supply, whichapplies a high energy electrical impulse between the foil strips. Ascurrent is conducted through the unwanted windings, as above, theresistance of the fine wire is sufficient to cause rapid heating andevaporation or disintegration of the unwanted turns.

Any wire fragments remaining after the electrical discharge step arefixed to the foil strips which can then be easily removed, leaving abare mandrel. The end of the wire remaining on the mandrel is alsobonded to the strip but can be easily broken without damage to thewinding.

Obviously, the invention can be practiced with individual segments offoil or with a continuous strip which can be moved to a fresh segmentafter each wound wire segment has been operated upon. Further, while theparallel gap welding step is not necessary, it has been found to bedesirable and effective. It is therefore employed in the preferredembodiment of the present invention and is recommended when practicingthe method.

Accordingly, it is an object of the present invention to provide animproved method for removing unwanted .tums from a wound wiredevice suchas a potentiometer coil.

It is yet another object of the invention to provide an improvedapparatus for removing unwanted turns from and advantages thereof willbe better understood from the following description considered inconnection with the accompanying drawings in which several preferredembodiments of the invention are illustrated by way of example. It is tobe expressly understood, however, that the drawings are for the purposeof illustration and description only and are not intended as adefinition of the limits of the invention.

FIG. 1 is an idealized view of apparatus for practicing the presentinvention;

FIG. 2 including FIGS. 2a and 2b is a side view of a potentiometer coilwith a partially broken away section enlarged to show detail;

FIG. 3 is an idealized end view, partly in block diagram, illustrating apreferred embodiment of the present invention employing both resistanceand parallel gap welding techniques;

FIG. 4 is a side sectional view of the apparatus of FIG. 3 taken alongthe line 44 in the direction of the appended arrows;

FIG. 5 is an idealized end view illustrating an alternative embodimentof the present invention;

FIG. 6 is an end view of a wound wire device as an electrical dischargeis passed through the undesired windings; and

FIG. 7 is a plan view of a segment of the ribbon of FIG. 3.

Turning first to FIG. 1 there is shown a wound wire device such as apotentiometer coil 10. The device includes a mandrel 12 which may be aductile material such as copper. Helically wound around the mandrel 12is a relatively fine resistance winding 14 which may be a varnishedcopper wire whose diameter is very small relative to the mandrel 12. Apair of electrodes l6, 18 are positioned adjacent the unwanted turns ofresistance wire 14. The electrodes l6, 18 are connected across a powersupply 20 which may be a conventional resistance welding power supply. Apushbutton 22 is shown as the means by which an electrical discharge canbe initiated across the electrodes l6, 18.

In operation, the wound wire device 10 is placed with the unwanted finewire 14 turns between the opposing electrodes 16, 18. The power supply20 is connected and the pushbutton 22 is actuated. A high energy,electrical discharge is conducted between electrodes 16, 18 by the turnsof wire 14 which are in contact with the electrodes. The electricalenergy through the fine wire 14 is sufficiently great to vaporize ordisintegrate substantially all of the wire 14 between the electrodes 16,18, leaving fragments or bits of the wire as indicated by the dottedsegment lines. These fragments remain because the electrodes 16, 18 alsofunction as a limited heat sink, and the fragments are thus notevaporated by the discharge. Removal of the electrodes l6, l8 generallyremoves the wire fragments, as well.

In FIG. 2a there is shown a side view ofa typical segment of apotentiometer coil 10', illustrating a helical winding of a very fineresistance wire 14 on a mandrel 12'. Th encircled area is enlarged inFIG. 2b to show additional detail.

In FIG. 2b, the relative diameters of the mandrel l2 and the fine wire14"can be appreciated. Generally,

the fine wire 14 is insulated with a varnish to afford electricalisolation between adjacent turns of the fine wire 14 and between thefine wire 14' and the mandrel 12'. An electrical potential is appliedbetween the ends of the coil 10. An area extending in the axialdirection is cleaned of insulation and a contact can pick off apotential that is related to the relative location of the contact.

Turning next to FIG. 3, there is shown a preferred embodiment of thepresent invention. As in FIG. 1, a wound wire device 30 includes amandrel 32 and a fine wire 34 which is helically wrapped about themandrel 32. In the preferred embodiment, two pairs of electrodes areemployed. A first opposed electrode pair includes a first electrode 36and a second electrode 38.

A third electrode 40 and a fourth electrode 42 form a second, opposedpair. A parallel gap welder power supply 44 is connected across theadjacent first and third electrodes 36, 40 of the different pairs. Asecond, similar parallel gap welding power supply 44' is connectedacross the second and fourth electrodes 38, 40 of the different pairs.It is, of course, possible that a single power supply 44 could beutilized and the adjacent electrodes would be connected in parallel.

A second, resistance welding power supply 46 is connected across atleast one of the pairs of electrodes, here shown as the pair includingthe first and second electrodes 36, 38. Between the wound wire device 30and the electrodes, continuous foil strips 48, 48' are placed.

Each of the foil strips 48, 48' includes a base ofa fair conductor suchas stainless steel, Kovar or the like. Bonded or laminated to the baselayer is a thin coating ofa brazing material such as gold which has arelatively low melting point and which can be employed in a brazingoperation. The ribbons 48, 48' are placed with the brazing metalsadjacent the wound wire device 30.

This is best shown in FIG. 4 which is a side section of the apparatus ofFIG. 3. As illustrated, the foil ribbons or strips 48, 48 are of a widthsubstantially equal to the width of the wound portion which is to beremoved. As shown, approximately four turns are to be removed.

In practicing the process of the present invention, and with referenceparticularly to FIGS. 3 and 4, initially the opposing pairs ofelectrodes are brought ad jacent the respective foil strips 48, 48' andslight pressure is applied, holding the foil 38, 38' against the finewire 34. The parallel gap power supplies 44, 44' are energized and acurrent is passed between the first and third electrodes 36, 40, and thesecond and fourth electrodes 38, 42.

This passage of electrical current causes local heating in the portionof the foil 48, 48 that bridges between the adjacent electrodes andcauses localized heating, melting the brazing material and effectivelybonding the foil to the fine wire turns directly underlying the foil.The power supplies 44, 44' are then disconnected, and the resistancewelding power supply 46 is connected and then energized. A high energyelectrical discharge passes between the opposed electrodes, carriedsubstantially by the turns of wire that have been bonded to the foilstrips 48, 48'.

The resistance welding power supply 46 provides a short impulse of highcurrent electrical energy which is sufficient to evaporate the completeturns of wire between the foil strips 48, 48'. However, because of theheat sink properties of foil strips 48, 48' and the electrodes withwhich they are in contact, fine wire fragments may be left which havebeen brazed to the foil strips 48, 48'. Further, one turn of the windingis bonded to only one of the foil strips and this one wire must bebroken. However, the tensile strength of the fine wire 34 is such thatthe single turn can easily be broken without deleterious effect to thewound wire device 30.

The pressure on the pairs of electrodes may then be relaxed and thewound wire device 30 removed. The

foil 48, 48 can then be advanced slightly to a fresh position and theopposite end of the wound wire device 30 can be similarly dressed toremove the unwanted end turns.

In an alternative embodiment, indicated in FIG. 5, the continuous foilribbons 48, .48 can be replaced with individual foil pieces of preciselydetermined width which can be placed under the electrodes before anypressure is applied. However, it has been found that the use of thecontinuous foil strips 48, 48' enables a faster, more repeatable processand avoids the need for the precise placement of the foil elements.

In FIG. 6, there is indicated the vaporizing process showing a fine wireturn to which a resistance welding discharge has been applied. Asis wellknown, the wire is rapidly heated to incandescence. Because of themagnitude of the discharge, substantially the entire wire issimultaneously heated to vaporization, leaving only fragments 50 at theelectrodes. This is better illustrated in FIG. 7, which shows a segmentof foil strip 48 having bonded thereto a plurality of fragments 50,remaining after the resistance welding discharge.

It has also been found that a ribbon strip intermediate the resistancevwelding electrodes in a work piece has the added advantage of protectingthe electrode from undue heating or burning during a discharge andeliminates the need for frequent cleaning and reshaping of the weldingtip. In such an application, the foil need not include the lamination ofbrazing material. It is only necessary to provide a conductive strip,which, for this application, may be a good conductor such as copper orsilver to assure that none of the electrical energy required for theresistance welding operation is lost in heating the interposed strips.

As has been noted above, the preferred embodiment utilizes a parallelgap welding step and a resistance welding step with an extra pair ofopposed electrodes added for accomplishing this method. It is also notedthat a satisfactory, but less desirable method could be performed withonly the single pair of opposed electrodes employing foil strips,wherein the single, resistance welding electrical discharge accomplishesthe vaporization of the unwanted fine wire turns and, at the same time,bonds the remaining fragments to the foil strips. It will therefore beapparent to those skilled in the art that modification and variationsare possible without departing from the basic teaching of the presentinvention.

What is claimed as new is: 1. A method of removing turns of fine wirewhich have been wound on a mandrel comprising the steps of:

positioning electrodes on opposed sides of the mandrel with electrodesin contact with one or more turns of the winding of fine wire; applyinga short duration electrical impulse between said electrodes sufficientfor interrupting all turns intermediate said electrodes; and removingall interrupted turns.

2. The method of claim 1, above, further including the steps of:

positioning additional electrodes on opposed sides of the mandreladjacent to electrodes already positioned;

placing a foil ribbon between electrodes and the turns to be removed;and

applying a first relatively low powered welding current between adjacentelectrodes to weld said ribbon to the unwanted turns, prior to applyingsaid short duration electrical impulse.

3. The method of claim 2, above, wherein said removing step includes thestep of removing said foil with fragments of unwanted interrupted turnsadherent thereto.

4. Apparatus for removing turns of fine wire wrapped on a mandrelcomprising in combination:

a. electrode means including electrodes spaced apart and in contact withturns to be removed, and

b. power supply means coupled to said electrode means for applyingelectrical energy between said electrodes of power sufficient tointerrupt all turns in contact with said electrodes, whereby unwantedturns on the mandrel are removed by an electrical discharge through theunwanted turns.

5. The apparatus of claim 4, above, further including contacting meansincluding conductive foil segments adapted to be interposed between saidelectrodes and the turns to be removed, whereby electrical energyapplied through said foil segments bonds the turns to said foil for easyremoval thereof.

6. The apparatus of claim 5, above, wherein said electrode means includeadditional electrodes adjacent said spaced apart electrodes, said foilsegments bridging adjacent electrodes, and wherein said power supplymeans include a secondary power supply connected across adjacentelectrodes for applying parallel gap welding currents through said foilsegments for bonding the turns to said segments.

7. Apparatus for removing a predetermined width of wire wrapped on amandrel, comprising in combination:

a. a first pair of opposed electrodes adapted to receive the wire woundmandrel therebetween;

b. a second pair of opposed electrodes next adjacent the electrodes ofsaid first pair and adapted to receive the wire wound mandreltherebetween;

c. conductive foil means including foil segments interposed between saidelectrodes and the wire wound mandrel;

d. first power supply means coupled to adjacent electrodes of said firstand second pairs for applying a parallel gap welding current throughsaid foil 'segments for bonding said foil segments to the wrapped wire;and

e. second power supply means coupled to at least one of each of saidpairs of electrodes for applying a resistance welding current to a pathincluding the wrapped wire between the opposed electrode of said pair,

whereby the resistance welding current interrupts the turns of wirebetween said opposed electrodes.

1. A method of removing turns of fine wire which have been wound on amandrel comprising the steps of: positioning electrodes on opposed sidesof the mandrel with electrodes in contact with one or more turns of thewinding of fine wire; applying a short duration electrical impulsebetween said electrodes sufficient for interrupting all turnsintermediate said electrodes; and removing all interrupted turns.
 2. Themethod of claim 1, above, further including the steps of: positioningadditional electrodes on opposed sides of the mandrel adjacent toelectrodes already positioned; placing a foil ribbon between electrodesand the turns to be removed; and applying a first relatively low poweredwelding current between adjacent electrodes to weld said ribbon to theunwanted turns, prior to applying said short duration electricalimpulse.
 3. The method of claim 2, above, wherein said removing stepincludes the step of removing said foil with fragments of unwantedinterrupted turns adherent thereto.
 4. Apparatus for removing turns offine wire wrapped on a mandrel comprising in combination: a. electrodemeans including electrodes spaced apart and in contact with turns to beremoved, and b. power supply means coupled to said electrode means forapplying electrical energy between said electrodes of power sufficientto interrupt all turns in contact with said electrodes, whereby unwantedturns on the mandrel are removed by an electrical discharge through theunwanted turns.
 5. The apparatus of claim 4, above, further includingcontacting means including conductive foil segments adapted to beinterposed between said electrodes and the turns to be removed, wherebyelectrical energy applied through said foil segments bonds the turns tosaid foil for easy removal thereof.
 6. The apparatus of claim 5, above,wherein said electrode means include additional electrodes adjacent saidspaced apart electrodes, said foil segments bridging adjacentelectrodes, and wherein said power supply means include a secondarypower supply connected across adjacent electrodes for applying parallelgap welding currents through said foil segments for bonding the turns tosaid segments.
 7. Apparatus for removing a predetermined width of wirewrapped on a mandrel, comprising in combination: a. a first pair ofopposed electrodes adapted to receive the wire wound mandreltherebetween; b. a second pair of opposed electrodes next adjacent theelectrodes of said first pair and adapted to receive the wire woundmandrel therebetween; c. conductive foil means including foil segmentsinterposed between said electrodes and the wire wound mandrel; d. firstpower supply means coupled to adjacent electrodes of said first andsecond pairs for applying a parallel gap welding current through saidfoil segments for bonding said foil segments to the wrapped wire; and e.second power supply means coupled to at least one of each of said pairsof electrodes for applying a resistance welding current to a pathincluding the wrapped wire between the opposed electrode of said pair,whereby the resistance welding current interrupts the turns of wirebetween said opposed electrodes.